a) Field of the Invention
The invention is directed to an image display system with a projector for emitting projection light for generating an image on a light-backscattering projection surface.
b) Description of the Related Art
Image display systems have been common for a long time and are used, for example, in slide projection applications in the home or for showing films in cinemas.
In the last thirty years, image display systems have been developed in which LCD projectors, DLP projectors or laser projectors are used for writing an image with a light bundle.
Known image display systems use light-backscattering projection surfaces to present images, wherein the diffuse reflectance of commercially available projection surfaces in the visible wavelength range is largely independent from wavelength. Such projection surfaces are provided with a frame and a stand or suspension and are also referred to as image screens or projection screens. Projection screens are standardized, for example, according to DIN 19045.
The projection surface diffusely reflects (backscatters) the light originating from the projector back to the observer who visually perceives an image.
When ambient light acts upon the projection surface, it is also scattered from the projection surface toward the observer. As a result, even lowintensity ambient light drastically worsens the contrast (light/dark ratio) and color saturation (colorfulness) of the projected image. The ambient light interfering with the image display is usually sunlight or artificially generated light corresponding at least in some proportion to sunlight. In extreme cases, the projected image is practically entirely obliterated, for example, when sunlight strikes the projection surface directly when projecting images.
In order to overcome this problem, images are preferably projected at night or in very dark rooms as is known, for instance, in cinemas or planetaria. Also, presentations in conference halls or schools require rooms offering at least the possibility of darkening. The darkening of rooms requires that the building be outfitted with special lighting equipment and light openings which must be closed when necessary so as not to admit any light. However, the disadvantage in darkened rooms is that many activities can no longer be performed without sufficient illumination, e.g., writing. For this reason, it has always been necessary heretofore, e.g., in the case of presentations, to find a compromise between the necessary ambient light and the quality of the image display.
Recently, the light output of projectors has been considerably augmented, particularly by means of more efficient light sources. This has entailed a considerable expenditure in material and energy, although only a very limited improvement in contrast and color saturation has been achieved. Up to the present, daylight projection is possible only to a very limited extent even with very lightintensive projectors, but is increasingly in demand for purposes of advertisement and presentation without any satisfactory solution being available so far.
In order to reduce the influence of ambient light on the quality of the projected image, WO 98/36320 describes a projection screen comprising a plurality of layers. The uppermost layer or sequence of layers, considered in the projecting direction, reflects only the projection light. All other light components are absorbed by a lower layer. A projection surface of this type has a substantially dark effect in ambient light and is therefore known as a xe2x80x9cblack wallxe2x80x9d. A visibly improvement in contrast would be expected with a projection screen of this type when using laser light for projection. However, this improvement is not yet so significant that the demand for projection in a conventionally illuminated environment has been met.
U.S. Pat. No. 2,590,906 or U.S. Pat. No. 5,568,316 describe interference filters which, as narrow-band reflection filters, are adapted exactly to the wavelengths of the laser light source, e.g., they can be used in connection with a projection screen known from WO 98/36320.
On this basis, it is the object of the invention to provide an image display system with a projection screen with which an image can be displayed whose contrast and color saturation is substantially retained even in comparatively intense ambient light. For this purpose, the projection screen should also be suitable for large-image display and should enable large projection angles and large observation angles. The invention has the aim of achieving a substantial improvement in the colorfulness (color saturation) of colors and in the contrast of a projected image perceived by the observer even when illuminated by artificial light and/or sunlight.
According to the invention, this is achieved by an image display system with a projector for emitting projection light for generating an image on a light-backscattering projection surface, wherein the wavelengths of the projection light lie in one or more discrete wavelength regions in the visible spectrum and the projection surface is covered on the projector side by an optical filter which passes only light with wavelengths corresponding to those of the projection light.
It is completely surprising for the person skilled in the art that the projection characteristics of contrast and color saturation of a projected image can be maintained virtually unaffected by the influence of the ambient light by covering the light-backscattering projection surface on the projector side with an optical filter layer of this kind.
In contrast, the basis of the known solution according to WO 98/36329 is that, initially, the projection light is essentially reflected in an upper layer, while the ambient light is transmitted through this upper layer into a lower layer and is absorbed in the latter.
However, in the invention the ambient light is prevented from passing through the top layer. Only the projection light reaches the light-backscattering projection surface. This surface lies below (considered in the direction of incident light) the blocking layer formed by the optical filter. Accordingly, practically no light ambient light, or only very little ambient light, reaches the surface on which the image display is realized.
It must be taken into account that the formation of a projected image is attributed not only to an interaction of the projection light with the surface of the projection surface, but that the projection light also, as a rule, can penetrate into the surface layer of the projection surface, where it is subjected to a volume scattering. The light that is backscattered as a result of the volume scattering reemerges at the surface of the projection surface as backscattered light.
The invention follows a completely novel path in that the ambient light is maintained at the same distance, as far as possible, from the projection surface which backscatters the light toward the observer.
A further advantage of the image display system according to the invention is that ambient light components which should penetrate the optical filter due to the fact that its blocking effect is only finite, impinges again on the optical filter after being backscattered at the projection surface, so that its filtering effect for the ambient light is multiplied. Therefore, in the invention, the contrast and color saturation of the image projection is degraded only to an insignificant degree by the ambient light.
A significant advantage of the invention is that the existing, standardized and commercially available projection screens can also be used as a basic component in the projection surface according to the invention.
In contrast to the xe2x80x9cblack wallxe2x80x9d known from the prior art, the invention can select from a large number of existing xe2x80x9cwhite screensxe2x80x9d with their specific characteristics.
In the invention, the spectrum of the projection light can be determined according to the image to be displayed. For example, the image may be a monochromatic image or a polychromatic image. The components of the image display system according to the invention are then configured corresponding to the selected wavelength range or the selected wavelength ranges.
In a preferred construction of the invention, the wavelengths of the projection light are in a wavelength range with a spectral bandwidth of less than 260 nm, in particular less than 80 nm. A spectral bandwidth of less than 260 nm, particularly with monochromatic image display, forms a limit, while a spectral .bandwidth of less than 80 nm is advisable particularly for displaying a polychromatic image.
In practice, however, the wavelength regions are no longer restricted, which results in a substantial improvement in image quality in relation to the influence of ambient light.
It is particularly preferred for monochromatic applications that the spectral bandwidth of the projection light is adjusted to the color to be projected and is at most about 120 nm. For displaying a red image, for example, as a warning, a light source with a radiation output maximum in the range of 620 nm to 630 nm is preferably selected.
Special advantages are realized through the use of a light source or light sources in the projector for generating projection light which particularly preferably emit(s) projection light with wavelengths in the ranges of 610 nm to 630 nm (red), 510 nm to 550 nm (green), and 400 nm to 470 nm (blue).
It is preferably provided that the transmission of the optical filter also achieves three maxima within these three wavelength ranges and is kept as small as possible outside of these wavelength ranges.
Because of its underlying physical principle, a laser light source generates a collinear light bundle with a small spectral bandwidth in the range of several tenths of a nanometer to several nanometers. However, relatively broadband laser radiation sources are advantageous in projection arrangements for speckle suppression, wherein a bandwidth of the laser radiation source between about 2 nm and 10 nm is particularly advantageous in connection with the invention. In this regard, it is also advantageous when the projection surface has good volume scattering characteristics in addition to the characteristic of diffuse reflectance.
The invention can also be used in particular for projecting color video images. The latter are generated from the three primary colors, red, green and blue. In this case, it is particularly favorable when the spectral half width is less than 10 nm for each of the primary colors. For color image display, an individual laser light source can be used which can generate the projection light in the three primary colors, red, green and blue, in a beam path (see, e.g., EP 0 788 015 A2) or which generates the projection light in separate beam paths (see, e.g., WO 91/12556 A1). Further, one laser light source can be used for generating projection light in each wavelength range which can be associated with one of the primary colors red, green and blue, respectively (as in WO 95/03675, for example).
However, the principle realized by the invention is not limited to the use of a laser light source for generating projection light. On the contrary, the projector can easily be constructed with a light source in the form of a temperature radiator, a gas discharge lamp or a semiconductor emitter, insofar as these light sources either emit only correspondingly narrow-band wavelength ranges or such narrow-band wavelength ranges are generated by correspondingly dimensioned filter elements.
A particularly advantageous construction of the invention consists in that the optical filter absorbs the incident light outside of its transmission range. This is advantageous in that the ambient light components are converted into heat energy and can no longer occur as light components in the eye of the observer.
However, it can also be advantageous when the optical filter in the invention has the characteristic that it reflects the incident light outside of its transmission range. In particular, an interference filter possesses this characteristic and also has the further advantage that it can be adapted very precisely to the wavelengths of the projection light and can be constructed with a very narrow band.
The reflection of the ambient light is carried out from the direction of the projection surface toward the observer and it would be expected that the reflected ambient light would lead to a deterioration in contrast and color saturation. Surprisingly, however, an entirely different result was shown. In spite of the fact that the ambient light is reflected back virtually in its entirety at the optical filter, the projected image appears with extremely full contrast and variegated colors without significant interference of the reflected ambient light.
The optical surface arranged in front of the entire surface of the projection surface can be produced by many different techniques and arranged in front of the projection surface.
It is particularly favorable when the optical filter is applied directly to the projection surface as a layer or layer system. The following production processes can be selected, for example: vacuum deposition, sputtering, chemical deposition, lamination, sol-gel process or painting. In particular, these processes enable largearea coating of projection surfaces having dimensions of several meters.
Another advantageous construction of the invention also consists in that the optical filter is arranged as a layer or layer system on a substrate which is transparent to the projection wavelengths. Therefore, the production of the optical filter can be carried out completely independent from the production of the projection surface. Only one assembly work step is required, wherein the substrate with the optical filter is joined with the projection surface. In the simplest case, the substrate with the optical filter is simply placed in front of the projection surface; however, this can result in additional reflections at the interfaces with the surrounding air. Therefore, it is particularly advantageous when the substrate is brought into direct contact with the surface of the projection surface which can be carried out, for example, simply by pressing on by means of compressed air or by adhesion forces. Examples of known production methods for this purpose are gluing or lamination. Technological requirements or determined filter materials may make it desirable to arrange a plurality of substrates one behind the other, wherein a layer or layer system is applied to each of these.
Accordingly, a desired spectral transmission of the optical filter can be realized by means of the principle of superposition of a plurality of filter layers.
A glass, a polymer material or a sheet is preferably used as substrate material. The substrate should have dimensions corresponding to the size of the projection surface in order to prevent seams due to parts placed against one another.
As was described above, interference filters are particularly well suited as optical filters. In this case, the optical filter is preferably formed of a system of nonabsorbent dielectric, or absorbent and nonabsorbent thin layers whose thickness is in the order of magnitude of the wavelength of visible light. Large-area production of such layers is relatively expensive. However, the extremely high degree of improvement in contrast and color saturation justifies this additional expenditure in many cases.
Less expensive but equally advantageous is the construction of the optical filter as a compound filter, especially in the form of a gelatin filter or colored glass filter.
While current known compound filters do not achieve the extreme selectivity of interference filters, they are likewise favorably suited for achieving a considerable improvement in contrast and in color saturation in image projection.
A particularly economical construction of the invention which utilizes the advantages of interference filters as well as compound filters consists in that the optical filter is a combination of a compound filter and interference filter, wherein, in particular, the substrate carrying the thin layers is designed as a compound filter. The transparent wavelength range of the optical filter need not necessarily correspond exactly to the wavelength ranges of the projection light; it can also be smaller. But this leads to intensity losses in projection light and accordingly to a darker image.
In certain cases, it is advantageous when the transparent wavelength range or wavelength ranges of the optical filter is or are selected so as to be up to 50 nm greater than the wavelength range or wavelength ranges of the projection light.
The effect whereby the transmission of an interference filter shifts to shorter wavelengths as the incident angles of the projection light increase is known (see Bergmann Schxc3xa4tfer xe2x80x9cLehrbuch der Experimentalphysik [Textbook of Experimental Physics]xe2x80x9d, volume 3, 9th edition, OPTIK Verlag Walter de Gruyter, Berlin, N.Y., 1993, 678). Therefore, it is advantageous particularly when narrow-band interference filters are used when the wavelength range or wavelength ranges of the optical filter is or are increased by up to 25 nm in the direction of shorter wavelengths relative to the wavelength range or wavelength ranges of the projection light. This ensures that all light components in the projection light are comprised by the bandwidth of the optical filter. Further, with respect to the dimensioning of the transmission ranges of an interference filter and the spectrum of the projection light, the transmission of the interference filter can also depend on the polarization direction of the projection light. This is an added advantage particularly when a laser light source is used and can be taken into account in the design of the image display system.
The invention also relates directly to a projection screen for use with an image display system according to the invention having a light-backscattering projection surface according to the invention which is coated with an optical filter that passes light in one or more discrete wavelength ranges in the visible spectrum and blocks the other wavelength ranges of visible light in front of the backscattering projection surface.
The invention will be described in principle in more detail in the following by way of example with reference to the drawings.